THERMAL INSULATION CAN BE DEFINED AS RESISTANCE TO THERMALTRANSMITANCE OVER A GRADIENT OF TEMPERATURE
IT CAN ALS0 BE TERMED AS THERMAL INERTIA OR THERMAL RESISTANCE.
IT CAN BE EXPRESSED IN BTU PER HR SQ FT DEGREE F.
MODES OF TRANSFER OF HEAT
CONDUCTION, the most common means of heat transfer in solids,
CONVECTION the most common means of heat transfer in liquids and gases, is the transfer of heat via a combination of conduction and fluid flow..
RADIATIONRadiation is the only form of heat transfer that can occur in the absence of any form of medium – through a vacuum., a net transfer of heat occurs from the hotter materials to the cooler materials.
THUS ,
THERMAL INSULATION =1/HEAT TRANSFER COEFFICIENT
(VALUE RANGES FROM 0.85-2.98)
Specific heat describes a material's ability to store heat energy. The specific heat of concrete and masonry can generally be assumed to be 0.2Btu/lb·°F. )Heat Capacity (HC) is the amount of heat energy required to raise the temperature of a mass one degree Fahrenheit. Heat capacity is per square foot of wall area (Btu/ft2·°F) and includes all layers in a wall. For a single layer wall, HC is calculated by multiplying the density of the material times its thickness (in ft) times the specific heat of the material. HC for a multilayered wall is the sum of the heat capacities for each layer.
CEMENT CONCRETE FOR HEAT
Concrete has an inherent capacity (related to its mass) to absorb and store thermal energy. This quality is referred to as 'thermal mass'
Thermal mass is a property that enables building materials to absorb, store, and later release significant amounts of heat. Buildings constructed of concrete and masonry have a unique energy-saving advantage because of their inherent thermal mass. These materials absorb energy slowly and hold it for much longer periods of time than do less massive materials. The thermal mass of concrete has the following benefits and characteristics
Delays peak loads
Reduces peak loads
Reduces total loads in many climates and locations
Works best in commercial building applications Works well in residential applications
Works best when mass is exposed on the inside surface
Works well regardless of the placement of mass Mass
works well in commercial applications by delaying the peak summer load, which generally occurs around 3:00 pm to later when offices begin to close.
CEMENT CONCRET IN Summer
In summer, energy from direct sun and from warm circulating air is absorbed by the cooler concrete mass thus reducing the air temperature within the home. As the air temperature decreases in the evening, stored energy within the concrete mass re- radiates ~ providing consistent comfortable temperatures within the home. This cooling effect of thermal mass is especially beneficial in very warm climates. Eaves should be designed to shade windows from high angled summer sun and there should be sufficient opening windows to allow cross ventilation.
CEMENT CONCRETE IN Winter
Capturing the free energy of the sun is relatively simple with a concrete home. This energy is most efficiently captured if the sun shines directly onto concrete surfaces, although reflected radiation will also be absorbed by concrete surfaces not directly exposed to sunlight. Convection and conduction also play a part.
Solar gain can be achieved by maximising the glazing that faces north ( ± 20° off north is best) and using low insulation floor coverings such as tiles on a concrete slab. Coloured concrete systems are also ideal. Carpet will insulate the concrete floor slab, which reduces its ability to absorb solar energy. Likewise plasterboard lining on concrete walls will reduce solar gain compared to hardwall plaster. Eaves and verandas should not prevent winter sun penetrating the glazing.
Thermal Performance of High Mass [Concrete] Houses
ACCORDING TO Research work undertaken by the Cement and Concrete Association of New Zealand (started in 1997) into the benefits of building a house from concrete ARE
The amount of glazing, and its orientation to the sun, has a significant effect on the performance of a home.
The concrete building used 15.5% less energy than the identical timber one for similar comfort conditions.
The concrete house was more comfortable when a large window was fitted, the timber home overheated significantly.
The concrete home was more than 5oC cooler than ambient on a 30oC day, while the temperature inside the timber home approximated the outside temperature.
Overnight, the timber home was on average, 1 degree cooler than the concrete one.
The minimum temperatures for the concrete and timber buildings were 15.6oC and 12.8oC respectively.
The timber home required four times the shading needed by the concrete home (to control overheating).
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